In general, a construction machine such as an excavator includes a plurality of actuators for travelling or driving various working apparatuses and the plurality of actuators are driven by a working fluid discharged from a variable-displacement-type hydraulic pump driven by an engine.
Meanwhile, the output of the engine and the flow of the working fluid discharged from the variable-displacement-type hydraulic pump are controlled based upon a work load. One example of an apparatus for controlling the flow of the hydraulic pump is shown in FIG. 1.
Referring to FIG. 1, a general construction machine includes two main pumps P1 and P2 and one auxiliary pump P3 driven by an engine E. The main pumps P1 and P2 are constituted by variable-displacement-type pumps where the discharged flow varies depending on angles of swash plates 1a and 1b. In the case of the main pumps P1 and P2, gradient angles of the swash plates 1a and 1b are controlled by driving servo pistons 2a and 2b to control the flow.
The servo pistons 2a and 2b are driven by working fluids of the main pumps P1 and P2 where flowing directions thereof are controlled by the swash plate control valves 5a and 5b. The swash plate control valves 5a and 5b are changed by driving multi-step pistons 6a and 6b and the multi-step pistons 6a and 6b are driven by flow control pistons 7a and 7b. That is, the gradient angles of the swash plates 1a and 1b of the main pumps P2 and P2 are controlled by driving the flow control pistons 7a and 7b. 
Further, the flow control pistons 7a and 7b are driven depending on the flow discharged from electro proportional control valves 8a and 8b of which an opening rate is controlled according to a current amount which is a signal applied from a controller 9.
More specifically, a pressure sensor 10 is provided on each of hydraulic control lines of a joystick of the excavator and various travelling control devices (not shown). When a user controls the joystick and various travelling control devices, the pressure sensor 10 recognizes signals depending on motions thereof and transmits the signals to the controller 9. The controller 9 uses an inputted pressure sensor value and outputs a signal corresponding thereto, i.e., the current amount, to the electro proportional control valves 8a and 8b so as to control the opening rates of the electro proportional control valves 8a and 8b, and as a result, the discharge flows of the main pumps P1 and P2 are appropriately controlled.
However, in case where the pressure sensor 10 is abnormal, the pressure sensor 10 cannot accurately detect the motions of the joystick and the various control devices and a pressure sensor value that is incorrectly detected is inputted into the controller 9, and as a result, the discharge flows of the main pumps P1 and P2 are not accurately controlled. Therefore, the construction machine does not operate or operates erroneously. Further, even when the error of the pressure sensor 10 is recognized, the construction machine cannot but stop until repairs can be completed.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.